This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2000-206758, filed Jul. 7, 2000; and No. 2000-289987, filed Sep. 25, 2000, the entire contents of both of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a liquid crystal display device having thin film transistors for reducing leak current, and more specifically, relates to an active matrix liquid crystal display device having thin film transistors each of which is provided with a gate electrode made of shielding material.
2. Description of the Related Art
Some of liquid crystal display devices have thin film transistors as switching elements. FIG. 13 is a partly enlarged plan view of such a kind of the conventional liquid crystal display device, and FIG. 14 is an enlarged sectional view taken along XIVxe2x80x94XIV line. The liquid crystal display device is provided with a lower glass substrate 101 and an upper glass substrate 102 bonded to each other by a sealing member (not shown) formed in an approximately square frame and disposed therebetween. Liquid crystal 103 is sealed between the glass substrates 101 and 102 and inside the sealing member.
The lower glass substrate 101 is provided at predetermined portions on the upper face thereof with scanning lines 105 each including a gate electrode 104 and formed of shielding metal material. The scanning line 105 extends in a direction from left to right in FIG. 13. The lower glass substrate 101 provided with the gate electrodes 104 and the scanning lines 105 also has on the upper face a gate insulating film 106 formed of silicon nitride or the like. There is provided a semiconductor thin film 107 formed of intrinsic amorphous silicon on a portion of the upper face of the gate insulating film 106, which is placed directly upper the gate electrode 104. On the upper face of the approximately central portion of the channel length of the semiconductor thin film 107, there is provided a channel protection film 108 formed of silicon nitride or the like. The upper face of the side portions of the channel protection film 108 and the upper faces of source and drain regions of the semiconductor thin film 107 are covered with ohmic contact layers 109 and 110 formed of n-type amorphous silicon.
There are provided signal lines 112 each including a drain electrode 111 and formed of shielding metal material on the predetermined portions of the upper faces of the ohmic contact layer 109 and the gate insulating film 106. The signal line 112 extends in a direction from the upper portion to the lower portion in FIG. 13. The other ohmic contact layer 110 is provided on the upper face thereof with a source electrode 113 formed of shielding metal material. The above-mentioned members, i.e., the gate electrode 104, the gate insulating film 106, the semiconductor thin film 107, the channel protection film 108, the ohmic contact layers 109 and 110, the drain electrode 111, and the source electrode 113 constitute a thin film transistor 114.
There is provided on the gate insulating film 106 an overcoat film 116 so as to entirely cover the thin film transistor 114. The overcoat film 116 is provided on the predetermined portion of the upper face with a plenty of pixel electrodes 117 formed of transparent metal material such as ITO. Each of the pixel electrodes 117 is connected to the source electrode 113 of the corresponding thin film transistor through a contact hole 118 formed in the overcoat film 116. On the other hand, there are provided a black mask 119 and a counter electrode 120 which are formed of metal material such as chromium oxide with relatively low reflectivity, such that the upper glass substrate 102 contacts with them on the lower face. The black mask 119 is formed in a rectangular frame and provided to each of the pixel electrodes so as to enclose the pixel region. The frame is shown in FIG. 13 as a two-dot line, that is arranged to face the scanning line 105 including the gate electrode 104 and the signal line 112 including the drain electrode 111. There is normally provided a first alignment film on the overcoat film 116 and the pixel electrode 117 on lower glass substrate 101, a second alignment film on the counter electrode 120 formed on the glass substrate 102. FIGS. 13 and 14, however, omit these alignment films.
The relationship between the gate electrode 104 and the channel protection film 108 will be described below. The channel protection film 108 is formed by the photolithography technique with use of a mask formed of photoresist subjected to the exposure from the back side (back side exposure) with use of the gate electrode 104 as a mask and the exposure from the front side (front side exposure) with use of a mask (not shown) and then developed. In the backside exposure using the gate electrode 104 as a mask, the photoresist is formed in a size a little smaller than that of the gate electrode 104 in area since light comes inside from the peripheral portion of the gate electrode 104 and the scanning line 105. FIG. 13 shows a value A (a distance between the edge [front edge] of the gate electrode 104 in the direction of the channel length and the edge [front edge] of the channel protection film 108 in the direction of the channel length) and a value B (a distance between the edge [side edge] of the gate electrode 104 in the direction of the channel width and the edge [side edge] of the channel protection film 108 in the direction of the channel width). They are both set around 2-3 xcexcm, normally.
If the conventional liquid crystal display device as mentioned above is used as a transmissive display, however, light incident from the backlight (not shown) arranged on the lower face of the lower glass substrate 101 is reflected by the blackmask 119. The reflected light transmits through the channel protection film 108 and reaches the semiconductor thin film 107, resulting in the increase of the leak current. On the other hand, it is required now to reduce in width the scanning line 105 including the gate electrode 104 and the signal line 112 including the drain electrode 111 in order to increase the aperture ratio of the liquid crystal display device and form the device more finely. If the values A and B are set at 2-3 xcexcm as mentioned above, however, the device cannot have larger aperture ratio or be formed more finely. If the values A and B are set smaller, however, the leak current will increase to deteriorate the OFF-performance of the thin film transistor 114, with the result that the display performance will be adversely affected.
The present invention is intended to provide possibility of reducing the leak current from the thin film transistor due to the reflected light transmitting through the channel protection film and reaching the semiconductor thin film of the transistor.
The liquid crystal display device according to an aspect of the present invention comprises a liquid crystal display device comprising: pixel electrodes formed of transparent conductive material; thin film transistors each provided with a gate electrode formed of shielding material and having a front edge, a rear edge, and a pair of side edges, a gate insulating film, a semiconductor thin film, a drain electrode, a source electrode, and a channel protection film smaller than the gate electrode, a thickness of the semiconductor thin film being at least 200 xc3x85 and less than 400 xc3x85, each of the pixel electrode having a edge adjacent to the front edge and one of the side edges of the gate electrode, the source electrode extending over the front edge of the gate electrode to be connected with the pixel electrode, the channel protection film having front edge facing the front edge of the gate electrode, and a distance between the front edge of the gate electrode and the front edge of the channel protection film being 0.2-1.2 xcexcm.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.